The present invention relates, in general, to a novel masking process. The process is similar to the single crystal reactive etch and metallization (SCREAM) process described in U.S. Pat. No. 5,198,390, although the present inventive process includes additional novel steps resulting in a powerful process useful in the fabrication of ultra-high aspect ratio, wafer-free, single crystal silicon, movable micromechanical devices and frame structures of large vertical depth and narrow linewidth.
The SCREAM process generates a method to integrate metal electrodes for capacitor actuators and thin film insulators for electrical isolation. This process is adapted to make operational micro-electromechanical as well as micro-opto-electromechanical mechanisms. The SCREAM process includes selective oxidation of narrow beam segments, formation of silicon-on-insulator structures for electrical isolation, and metal contact incorporation on the suspended silicon beams. Sputtered metal is selectively etched from all the mirrors or optical components of the device. A final isotropic etch selectively releases the moving structures and the oxide isolating segments mechanically connect, but electrically isolate the silicon beams.
The present invention shows Single Crystal Silicon (SCS) to be an excellent base from which to build high-aspect-ratio micromechanisms. SCS exhibits low defect density, low internal friction, and high fatigue strength. However, it is difficult to develop high aspect ratio SCS processes that allow formation of complex geometry, freely suspended, submicron structures. The etch rate of wet chemical etching is usually highly dependent on crystal orientation, consequently larger SCS structures are more easily fabricated if the minimum feature size and feature spacing are compatible with tolerances of the etch-dependent irregular surfaces. Such large (&gt;10 .mu.m) chemically etched structures have been utilized to make accelerometers and pressure sensors. SCS is also highly transparent in the near infrared (IR) region for wavelengths greater than about 1 .mu.m, which makes it ideal for micro-opto-electromechanical device applications. Also, the etch masks used in SCS processing such as SiO.sub.2 and Si.sub.3 N.sub.4 can be used as thin-film optical coatings since they also exhibit very low energy absorption in the near infrared region.
Recent trends in the field of micromechanics are leading to larger surface to volume ratio SCS structures for the generation of large forces and displacements. Thus, high aspect ratio capacitor drives of large vertical depth to lateral width are needed for the actuation of such devices. A popular but expensive way to achieve the fabrication of deep structures is to use the LIGA technique (in German: Lithographie, Galvanoformung, Abformung). This process uses X-ray lithography, electroforming and molding of plastics, ceramics, metals and metal alloys and generates microstructures with structural heights of several 100 .mu.m and lateral dimensions of a few micrometers with an accuracy in the submicrometer range. Synchrotron radiation offers the advantage of high energy density and excellent parallelism at short wavelengths (0.2 to 0.5 nm). However, this LIGA process is not accessible to most research groups and industries because of its high cost. Other complex methods include using cryogenic dry etching at very low temperatures (-120.degree. C.) for maximum etch rate of silicon, or electroplating of high aspect ratio resist profiles.
Up until recently, SCS bulk micromachining did not offer such deep trench etching capabilities, and even in the extreme cases, maximum depths of no more than about 30 .mu.m have been achieved, the limitation being the practical thickness of the etch mask (whose profile is directly transferred into the etched substrate). During a deep trench etch, the selectivity (measure of the preferential etch rate of the substrate relative to that of the mask), is not high enough and the mask erodes away. Its thickness is limited by the practical amount of masking material that can be deposited, something less than 5 .mu.m.
Therefore, deep, vertical and smooth etch masks are highly desirable to produce high aspect ratio structures. Large surface to volume structures also imply narrow linewidth devices and there is a need for making beam widths from 2 .mu.m down to a 1/10 .mu.m.
The integration of microelectromechanical systems (MEMS) with optical devices resulting in micro-opto-electromechanical systems (MOEMS) offers many new possibilities in the fields of micromechanics and micro-optics.
For example, a key component of Wavelength Division Multiplexed (WDM) networks is a tunable broadband wavelength demultiplexer with good selectivity. The function of a wavelength division demultiplexer is to spatially separate n channels according to wavelength. This selection may be done in parallel (n channels directed to n detectors simultaneously), or serially (one of n channels directed to a single detector). In general, parallel elements utilize multiple fiber devices, while serial elements utilize tunable devices. The Fabry-Perot interferometer provides sharp, low-loss and narrow linewidth optical transmission peaks capable of being tuned to select a particular wavelength channel from a light source such as a distributed feedback (DFB) laser while providing isolation to all other operating channels. These characteristics offer powerful communications applications to optical fiber WDM system technology.
Tunable laser sources can also be generated by using microelectromechanically modulated Fabry-Perot interferometers as laser intra-cavity elements (e.g. movable mirrors for a doubly-resonant optical parametric oscillator OPO (DRO)), or as filter elements. A tunable ring laser setup, including a semiconductor amplifier, an optical isolator and a polarization controller, has been described for such a filter application.
In-the-plane, micromachined Fabry-Perot interferometers have already been investigated by other research groups. The device fabrication consisted of bonding two silicon wafers together, one with movable central elements, and including a thin etch-stopped corrugated diaphragm as the suspension. The present invention results in the fabrication of an out-of-plane SCS Fabry-Perot interferometer. Using the SCREAM process alone, this device appears to be the first device of its kind using only one SCS wafer. The process produces self-aligned, released, movable vertical mirror elements. The modified version of this process, which is the subject of the present invention, can produce high-aspect ratio, vertical and smooth mirror sidewalls.
Microelectromechanical devices using comb drives such as accelerometers can be considered as a repeated sequence of three-mirror Fabry-Perot interferometers, and each of the three-mirror unit comprises in itself two interferometers in series with different gaps. The effective free spectral range (FSR) of such a three-mirror system would increase by about an order of magnitude compared to a two-mirror unit. Devices with comb drive geometries can be visualized as higher performance but more complex FPI's. In this manner, they could constitute sensitive displacement or acceleration sensors.
Light modulators are usually based on changes of the refractive index of the material due to electro-optics, acousto-optics or magneto-optics. With a micromechanical device, the width of the optical resonance cavity can be modulated electrostatically, thus changing the spectral reflectivity or transmissivity of the incident light. The Fabry-Perot interferometer can be considered as the archetype of the optical resonator. This optical element consists of two partially reflecting, low loss, parallel mirrors separated by a gap. The optical transmission characteristic through these mirrors consists of a series of sharp peaks with narrow bandwidth (BW) when the gap distance equals a half wavelength multiple of the incident light. The transmission intensity depends on both the gap distance and the wavelength. So, by varying the gap distance and monitoring the intensity, the device can be used as a wavelength tuner, within the free spectral range (FSR) of the device. Otherwise, by maintaining the incident wavelength fixed, the device can be used as a very sensitive displacement, acceleration or pressure sensor. An important feature of the FPI is that the FSR and the BW can be independently controlled. For a given wavelength, the cavity gap sets the FSR and the mirror reflectivity controls the bandwidth. Excellent wavelength tuning is contingent upon a high cavity finesse (the finesse of an interferometer can be associated with the quality factor Q of a classical oscillator).